Such a heat transport assembly is known from U.S. Pat. No. 6,651,732-B2. The known heat transport assembly is a heat dissipating assembly consisting of a heat generating device comprising a heat pipe component and a heat sink being a plastic part having an integral pocket for receiving the heat pipe component. The plastic part in the assembly of U.S. Pat. No. 6,651,732-B2 is an injection molded thermally conductive elastomeric part.
In the electronics and computer industries, it has been well known to employ various types of electronic device packages and integrated circuit chips, such as the PENTIUM central processing unit chip (CPU) manufactured by Intel Corporation and RAM (random access memory) chips. These integrated circuit chips generate a great deal of heat during operation, which must be removed to prevent adverse effects on operation of the system into which the device is installed. There are a number of prior art methods to cool heat generating components and objects to avoid device failure and overheating. Often, block heat sinks or heat spreaders are placed in thermal communication with the surface of the heat-generating device to absorb heat and to help dissipate heat therefrom.
It is well known that the contacting surfaces at the interface critically affect the overall performance of the heat transfer assemblies. A problem with existing heat transport assemblies is the following. Generally, due to manufacturing tolerances the contact surfaces are not always perfectly flat thus creating gaps between the heat generating surface and the heat dissipating devices and thereby increasing the overall thermal resistance of the assembly. Furthermore, surface irregularities, for example due to milling or other processing steps, create micro voids and gaps between the contacting parts. These imperfections and gaps between the contacting surfaces often contain small pockets of air. This all results in a bad heat conductive contact, reduction of the heat transfer potential across the interface between the heat generating surface and the heat-dissipating device, and leading to a sharp temperature gradient across the contacting interface. This reduction in heat transfer potential can be very critical for the performance of the heat transport assembly, in particular when one of the two contacting parts is a plastic part.
In the above cited U.S. Pat. No. 6,651,732-B2, the surface contact area between the heat generating device and the plastic heat sink part is enhanced by the heat pipe component comprised by the heat generating device integral pocket for receiving the heat pipe component comprised by the heat sink. This solution of enhanced surface contact area compensates for the bad heat conductive contact, but does not reduce or solve it.
In order to reduce the effect of the bad heat conductive contact and to minimize the resulting problem of the limited heat transfer potential, there have also been different attempts to bridge the interface gap with a thermally conductive material to provide an intimate contact between the surface of the heat sink and the surface of the heat generating source.
In particular, heat conductive pastes, films, adhesives consisting of organic base materials such as elastomeric rubbers, thermoplastic materials, pastes, oils and greases loaded with thermally conducting ceramics or other fillers have been used as thermal interface material. Thermally conductive pastes, oils and greases are typically applied by smearing the heat sink or other electronic component with the thermally conducting material and then securing the heat sink in place by mechanical means using clips or screws. Some of these materials show superior film forming and gap filling characteristics between uneven surfaces thus providing an intimate contact between the surface of the heat sink and the surface of the heat-generating source. These properties are generally combined with a low viscosity and/or low filler content, resulting in too high a thermal resistance and/or effectively seeping out from between the heat sink and the heat generating surface, causing air voids to form between the two surfaces, ultimately leading to hot spots. Moreover, excessive pressure placed upon the heat sink by the mechanical fasteners accelerates this seepage from between the heat sink and the surface of the heat-generating surface. Others show a high thermal conductivity due to high filler load, but generally also have very high viscosities and too low wetting and/or exhibit poor adhesion to the surfaces of the heat sink and heat generating surface, and are susceptible for voiding and drying out and ultimately leading to hot spots. Other problems are that some of the oils can evaporate and recondense on sensitive parts of the surrounding microcircuits. The recondensed oils lead to the deposite formation thereby interfering with the function of the microprocessor and eventually causing failure.
In the case of the heat conductive elastomeric rubbers and thermoplastic materials, these materials are typically cast in sheet form and die cut into shapes corresponding to the shape of the contact surface of the heat sink and heat generating device. The resulting preformed sheet is then applied to the surface of the contact surface of the heat sink or the heat generating device. The sheet must be very soft to replicate the contact surface of that part. For example, WO02/059965 describes a compressible phase change thermal interface, interposable between the heat transfer surfaces of a heat generating component and a thermally dissipating member. Often a heat conductive adhesive material is used in addition. Alternatively the layer of heat conductive material is cast directly onto the contact surface of either the heat sink or the heat generating device, thereby eliminating the need of heat conductive adhesive material. Than the heat sink and heat generating device with the interface surface layer of heat conductive material are secured by means of clips or screws. The interface surface layer consisting of a precast film or a precut film adhered to one part solves the problems associated with greases and alike, and generally has an intimate contact with the part onto which the film is adhered or cast.
However, to provide a good heat conductive contact with the second part, excessive pressure must be placed on the interface layer. Moreover, these types of materials do not provide adequate intimate contact with the second part required for optimum heat transference between the heat generating source and the heat sink in a heat transport assembly comprising a plastic part and/or show variable performance due to variation in the thickness of the thermally conducting precut or precast films and to the amount of pressure applied to the thermally conducting film, based upon the mechanical device or action used to secure the heat sink.
The aim of the invention is to reduce or even fully eliminate the above mentioned problems and to provide a heat transport assembly having a good thermal communication between at least one plastic part in heat conductive contact with another part, more particularly to lower the sensitivity of the assembly's thermal contact resistance for irregularities and roughness of the surfaces in thermal contact and for to make the assembly's thermal contact resistance low and less critical to micro-void formation while still having a good thermal communication between the two parts.
This aim has been achieved with the heat transport assembly comprising a first part being a plastic part and a second part, and having a first surface area on the first part in heat conductive contact with a second surface area on the second part, wherein the first surface area and the second surface area consist of a surface material having a heat conductivity of at least 50 W/m·K.